The demand for nanoscale components in integrated circuits, medical diagnostics, and optoelectronics has generated much interest in the development and study of various lithography strategies. Conventional patterning methods, however, have failed to satisfy the need for rapidly patterning of nanoscale features at a low cost. The expense of patterning equipment grows dramatically as the required resolution increases.
With conventional far-field optical lithography, lateral feature resolution is diffraction-limited, as defined by the Rayleigh or Abbé conditions, which in practical terms only allow feature dimensions of approximately half the incident wavelength. In order to overcome the diffraction limit, a number of lithography approaches have been reported, including multi-photon induced photoresist polymerization, zone-plate array lithography, and phase-shift photolithography. Though these techniques are highly parallel, they rely on non-standard optical instrumentation and light sources not readily available to most researchers, or they preclude arbitrary nanoscale pattern formation. In order to produce complex patterns, established approaches including electron-beam lithography, focused ion beam (FIB) lithography, and scanning probe microscopy (SPM)-based techniques such as dip-pen nanolithography (DPN) have been employed. Near-field scanning optical microscopy (NSOM)-based techniques and scanning near-field photolithography (SNP) are promising custom lithographic methods for sub-diffraction limit patterning, but are inherently low throughput and restricted to scan areas several hundred microns in length.
In order to generate sub-diffraction limit features, SNP optics rely on the evanescent field of incident light passing through an aperture, the intensity of which is strongly dependent on the distance between this aperture and the surface. To control precise aperture heights and lateral registry, SNP relies on feedback systems used in piezo-controlled SPM instruments. Though highly parallel two-dimensional (2D) silicon-based NSOM aperture arrays have been fabricated, aligning a large area substrate surface with near-field proximity to this hard, non-deformable aperture array remains challenging. As a result, no successful demonstrations of their use in homogeneous patterning have been reported.
Beam pen lithography (BPL) is another desktop fabrication technique, which uses light to write patterns, as opposed to electrons and other particle-based techniques. Near-field apertures in a BPL tip array offer a direct route to circumvent the diffraction limit present in conventional photolithography. However, BPL is limited in that all tips in the array act in unison, making this technique only useful for generating replicas of patterns, and the apertures are either constructed serially using a focused ion beam or in parallel using a mechanical stripping technique that yields large micron-sized pores.